Effects of electric field on the electronic structure and optical properties of quantum rods with wurtzite structure

The Hamiltonian of wurtzite quantum rods with an ellipsoidal boundary under electric field is given after a coordinate transformation. The electronic structure and optical properties are studied in the framework of the effective-mass envelope-function theory. The quantum-confined Stark effect is illustrated by studying the change of the electronic structures under electric field. The transition probabilities between the electron and hole states decrease sharply with the increase of the electric field. The polarization factor increases with the increase of the electric field. Effects of the electric field and the shape of the rods on the exciton effect are also investigated. The exciton binding energy decreases with the increase of both the electric field and the aspect ratio. In the end, considering the exciton binding energy, we calculated the band gap variation of size- and shape-controlled colloidal CdSe quantum rods, which is in good agreement with experimental results.

A partially incoherent rate theory of long-range charge transfer in deoxyribose nucleic acid

A quantum chemistry based Green's function formulation of long-range charge transfer in deoxyribose nucleic acid (DNA) double helix is proposed. The theory takes into account the effects of DNA's electronic structure and its incoherent interaction with aqueous surroundings. In the implementation, the electronic tight-binding parameters for unsolvated DNA molecules are determined at the HF/6-31G* level, while those for individual nucleobase-water couplings are at a semiempirical level by fitting with experimental redox potentials. Numerical results include that: (i) the oxidative charge initially at the donor guanine site does hop sequentially over all guanine sites; however, the revealed rates can be of a much weaker distance dependence than that described by the ordinary Ohm's law; (ii) the aqueous surroundings-induced partial incoherences in thymine/adenine bridge bases lead them to deviate substantially from the superexchange regime; (iii) the time scale of the partially incoherent hole transport through the thymine/adenine pi stack in DNA is about 5 ps. (C) 2002 American Institute of Physics.

Electronic structure and transport properties of quantum rings in a magnetic field

The electronic structure of quantum rings is studied in the framework of the effective-mass theory and the two dimensional hard wall approximation. In cases of both the absence and presence of a magnetic field the electron momenta of confined states and the Coulomb energies of two electrons are given as functions of the angular momentum, inner radius, and magnetic-field strength. By comparing with experiments it is found that the width of the real confinement potential is 14 nm, much smaller than the phenomenal width. The Coulomb energy of two electrons is calculated as 11.1 meV. The quantum waveguide transport properties of Aharonov-Bohm (AB) rings are studied complementarily, and it is found that the correspondence of the positions of resonant peaks in AB rings and the momentum of confined states in closed rings is good for thin rings, representing a type of resonant tunneling.

Electronic structure of diluted magnetic semiconductor superlattices: In-plane magnetic field effect

The electronic structure of diluted magnetic semiconductor (DMS) superlattices under an in-plane magnetic field is studied within the framework of the effective-mass theory; the strain effect is also included in the calculation. The numerical results show that an increase of the in-plane magnetic field renders the DMS superlattice from the direct band-gap system to the indirect band-gap system, and spatially separates the electron and the hole by changing the type-I band alignment to a type-II band alignment. The optical transition probability changes from type I to type II and back to type I like at large magnetic field. This phenomenon arises from the interplay among the superlattice potential profile, the external magnetic field, and the sp-d exchange interaction between the carriers and the magnetic ions. The shear strain induces a strong coupling of the light- and heavy-hole states and a transition of the hole ground states from "light"-hole to "heavy"-hole-like states.

Electronic structure of quantum spheres with wurtzite structure

The hole effective-mass Hamiltonian for the semiconductors with wurtzite structure is given. The effective-mass parameters are determined by fitting the valence-band structure near the top with that calculated by the empirical pseudopotential method: The energies and corresponding wave functions are calculated with the obtained effective-mass Hamiltonian for the CdSe quantum spheres, and the energies as functions of sphere radius R are given for the zero spin-orbital coupling (SOC) and finite SOC cases. The energies do not vary as 1/R-2 as the general cases, which is caused by the crystal-field splitting energy and the linear terms in the Hamiltonian. It is found that the ground state is not the optically active S state for the R smaller than 30 Angstrom, in agreement with the experimental results and the "dark exciton'' theory. [S0163-1829(99)01040-1].

The effects of electric field on the electronic structure of a semiconductor quantum dot

The effect of electric field on the electronic structure of a spherical quantum dot is studied in the framework of the effective-mass envelope-function theory. The dependence of the energy of electron states and hole states on the applied electric field and on the quantum dot size is investigated; the mixing of heavy holes and light holes is taken into account. The selection rule for the optical transition between the conduction band and valence band states is obtained. The exciton binding energies are calculated as functions of the quantum dot radius and the strength of the electric field. (C) 1998 American Institute of Physics.

Gaussian diffraction model for Sb thin films in super-resolution near-field structure

SPIE

EFFECTIVE-MASS THEORY FOR GAAS/GA1-XALXAS QUANTUM WIRES AND CORRUGATED SUPERLATTICES GROWN ON (311)-ORIENTED SUBSTRATES

The electronic structures of GaAs/Ga1-xAlxAs quantum wires (corrugated superlattices) grown on (311)-oriented substrates are studied in the framework of the effective-mass envelope-function method. The electron and hole subband structure and optical transition matrix elements are calculated. When x=1, the results are compared with experiments, and it is found that the direct transition becomes an indirect transition as the widths of well and barrier become smaller.

SCATTERING AND RESONANT-TUNNELING THEORY FOR A CYLINDRICAL WELL IN PLANAR STRUCTURES

A two-dimensional atomic scattering theory is developed for scattering of electrons by a circularly symmetric quantum structure in the two-dimensional electron gas. It is found that the scattering cross section oscillates as a function of ka where k is the electron wave vector and a is the radius of the cylindrical potential barrier. If there is a quantum well inside the potential barrier, there appears a series of sharp resonant-tunneling peaks superposed on the original scattering-cross-section curves. The width of the resonant-tunneling peak depends sensitively on the thickness, the height of the potential barrier, and the electron energy.

EFFECTIVE-MASS THEORY FOR SUPERLATTICES GROWN ON (11N)-ORIENTED SUBSTRATES

An effective-mass formulation for superlattices grown on (11N)-oriented substrates is given. It is found that, for GaAs/AlxGa1-xAs superlattices, the hole subband structure and related properties are sensitive to the orientation because of the large anisotropy of the valence band. The energy-level positions for the heavy hole and the optical transition matrix elements for the light hole apparently change with orientation. The heavy- and light-hole energy levels at k parallel-to = 0 can be calculated separately by taking the classical effective mass in the growth direction. Under a uniaxial stress along the growth direction, the energy levels of the heavy and light holes shift down and up, respectively; at a critical stress, the first heavy- and light-hole energy levels cross over. The energy shifts caused by the uniaxial stress are largest for the (111) case and smallest for the (001) case. The optical transition matrix elements change substantially after the crossover of the first heavy- and light-hole energy has occurred.

Effective-mass theory for InAs/GaAs strained superlattices

By using the recently developed exact effective-mass envelope-function theory, the electronic structures of InAs/GaAs strained superlattices grown on GaAs (100) oriented substrates are studied. The electron and hole subband structures, distribution of electrons and holes along the growth direction, optical transition matrix elements, exciton states, and absorption spectra are calculated. In our calculations, the effects due to the different effective masses of electrons and holes in different materials and the strain are included. Our theoretical results are in agreement with the available experimental data.

Electronic structure of quantum spheres and quantum wires

The electronic structures of quantum spheres and quantum wires are studied in the framework of the effective-mass theory. The spin-orbital coupling (SOC) effect is taken into account. On the basis of the zero SOC limit and strong SOC limit the hole quantum energy levels as functions of SOC parameter lambda are obtained. There is a fan region in which the ground and low-lying excited states approach those in the strong SOC limit as lambda increases. Besides, some theoretical results on the corrugated superlattices (CSL) are given.

Self-consistent calculation of electronic states in asymmetric double barrier structure

With contributions from both three-dimensional (3D) electrons in heavily doped contacts and 2D electrons in the accumulation layer, a self-consistent calculation based on effective mass theory is presented for studying the anomalous behaviour of the quasi-bound levels in the accumulation layer and that in the central well of an asymmetric double barrier structure (DBS). By choosing the thickness of the incident barrier properly, it is revealed that these two quasi-bound levels may merge into a unique bound level in the off-resonance regime which shows a very good 2D nature in contrast to the conventional picture for level crossing. An evident intrinsic I-V bistability is also shown. It is noticeable that the effect of charge build-up in the central well is so strong that the electric field in the incident barrier even decreases when the applied bias increases within the resonant region.

A novel polarization-insensitive semiconductor optical amplifier structure with large 3dB bandwidth

A novel approach to achieving a polarization-insensitive semiconductor optical amplifier is presented. The active layer consists of graded tensile strained bulk-like structure. which can not only enhance TM mode material gain and further realize polarization-insensitivity, but also get a large 3dB bandwidth due to different strain introduced into the active layer. 3dB bandwidth more than 40nm. 65nm has been obtained in die experiment and theory, respectively. The characteristics of such polarization insensitive structure have been analyzed, The influence of the amount of strain and of the thickness of strain layer on the polarization insensitivity has been discussed.